1. Field of the Invention
The invention refers to a method for detecting an inverter hardware failure in an electric power train.
2. Description of the Background Art
Hybrid vehicles are equipped with high power electric machines which are energized by high voltage batteries. The DC voltage of the battery is pulse-width-modulated by a number of power electronic switches as parts of a power inverter arranged in legs each one supplying a terminal of the electric machine. Two of the switches are connected in series in each leg and the electric machine's terminal is connected in the middle to both of them. The pulse-width-modulated voltage is integrated by the electric machine's inductance resulting in a virtual sine wave AC voltage. As in most cases the electric machine is a three phase machine and three inverter legs are provided and controlled so as to generate a three phase AC voltage respectively.
The power train composed of the electric machine, the inverter switches and respective high voltage cables are connected to the high voltage battery via a link, the link comprising a bulk capacitance and a Y-capacitance, both connected to a positive terminal and to a negative terminal of the high voltage battery. The Y-capacitance has three terminals, two of them connected to the positive terminal and the negative terminal of the battery. The third one is a center tapping connected to a metallic housing or chassis of the vehicle which is electrically isolated from the battery. The bulk capacitance serves for reducing voltage ripple, while the Y-capacitance is required to meet EMC (electromagnetic compatibility) and HV (high voltage) isolation requirements. It serves for reducing or eliminating common-mode voltage at the terminals of the electric machine caused by the machine's stray capacitance. A voltage divider arranged between the battery terminals provides a virtual mid-pack battery potential. A mid-pack voltage sensor is arranged between the metallic housing and the virtual mid-pack battery potential. The mid-pack voltage sensor is used for detecting isolation faults between the housing or chassis and the battery.
When the vehicle equipped with the electric power train has been stalled due to conditions pointing to a failure in the electric components of the power train, there are three major sources of the fault: 1) the electric machine, 2) the high voltage cables, or 3) the power inverter, i.e. at least one of the electronic power switches. An visual check of the high voltage cable can easily determine or eliminate it as the source of the fault. However, determining between the electric machine and the power inverter is not trivial and can cause unnecessary expenses. If there is no instrumentation available, there are only a few subjective inspections available before a mechanic or a technician will first replace the power inverter and then the transmission until the vehicle is running properly again.
In order to make sure the power train operates reliably, the power electronic switches, e.g. IGBTs, have to be tested.
U.S. Pat. No. 6,020,735 discloses an internal diode detection circuit for a device under test and a method of operating a test fixture for employing the same. The internal diode detection circuit includes a source of electrical power that applies a reverse bias voltage to the device under test. The internal diode detection circuit also includes a current indicator, series-coupled to the device under test that generates a signal when a current flows through the device under test. The signal indicates a presence of an internal diode in the device under test.
U.S. Pat. No. 6,145,107 discloses a method for early failure recognition in power semiconductor modules which employs a measurement across a resistor between a bonded emitter terminal and a bonded auxiliary emitter terminal that identifies the degradation of the bond point which triggers an early warning signal so that the power semiconductor module can be changed before failure and the overall reliability of an electronic power system can thereby be increased.
U.S. Pat. No. 5,895,989 discloses a power supply unit adapted to supply power from a power supply section to a load via a plurality of power supply lines connected by an on/off switch and connectors. The power supply unit includes a load open detection section for detecting an open state between the on/off switch and the load, a monitor section for monitoring the number of times the open state has been entered per unit time based on the detection result of the load open detection section, and a control section for controlling the on/off switch off when the number of times the open state has been entered per unit time is greater than a predetermined number of times.
It is an object of the present invention to provide a method for detecting an inverter hardware failure in an electric power train.